ASSESSING THE INCIDENCE OF INFECTION WITH PLASMODIUM-FALCIPARUM AMONGINTERNATIONAL TRAVELERS

Circumsporozoite (CS) antibodies, indicating plasmodial infection but not necessarily development of disease, have been shown to be reliable indicators of transmission in endemic areas. To estimate the actual r ate of plasmodial infection, the prevalence of CS antibodies was inves tigated by an ELISA test system in a selected population of 2,131 trav elers returning from areas endemic for malaria who presented to an out patient clinic without any apparent symptom or clinical sign of malari a. Serum specimens from 104 of the investigated 2,131 patients (4.9%) were found to be positive (titer greater than or equal to 6.25 interna tional ELISA units [IEU]). The geometric mean titer of antibody concen trations (IEU) in seropositive patients was 18.64 IEU (95% confidence interval [CI] = 13.15-24.13), while it was 2.1 IEU (95% CI = 1.8-2.4) in seronegative patients. A significantly above average risk for plasm odial infection could be found among travelers to East Africa (risk ra tio [RR] = 4.5, P < 0.001), West Africa (RR = 4.5, P < 0.001), and Sou thern Africa (RR = 3.2, P = 0.015), while areas with a comparatively l ow risk included Central America (RR = 0.86, P < 0.001), the Indian su bcontinent (RR = 0.45, P = 0.015), South America (RR = 0.49, P = 0.091 ), East Asia (RR = 0.6, P = 0.441), West Asia (RR = 0.24, P = 0.099), and Southeast Asia (RR = 0.69, P = 0.094). The results of this study e mphasize the importance of adequate malaria chemoprophylaxis in nonimm une travelers to endemic areas. By use of the described method, estima tes of the true infection rate of malaria in travelers can be derived for certain areas and the value of prophylactic measures can be demons trated.